1. Field of the Invention
The present invention is directed to a treatment process to stabilize ignitable characteristics of finely divided zirconium and produce an acceptable radioactive waste form.
2. Description of the Related Art
Untreated radioactive zirconium swarf is prohibited from land disposal under current Environmental Protection Agency (EPA) regulations due to its ignitable characteristic.
In-situ stabilization provides a means to treat the ignition hazards associated with zirconium swarf while producing a final radioactive waste form using a cost effective process.
Zirconium alloys are commonly used within the nuclear industry as a core cladding material. When the nuclear core has exceeded its useful life it is often cut-up and repackaged for storage or disposal processing Zirconium swarf generated as a by-product of the cutting operations is considered a mixed hazardous and radioactive waste. The swarf is hazardous due to its ignitable characteristic and is radioactive since it has been highly irradiated within the reactor.
When zirconium and zirconium alloys are finely divided, the ignitable propensity of the material is increased similar to magnesium and other reactive metals. Zirconium swarf generated by common cutting operations is small enough that it is often considered hazardous under EPA regulations (40 CFR 261.21(a)(2)). Since there are currently no facilities available to dispose of a mixed waste and because it is substantially impossible to remove the radioactivity from the material other than eventual decay, the ignitable characteristic of the material must be treated to render it non-hazardous. Once the hazardous characteristic has been treated, the waste is no longer classified as a mixed waste and it can be disposed of as a radioactive waste provided the waste form meets radioactive waste regulations (10 CFR 61).
Although work has been done to treat zirconium swarf by diluting it with cement grout, this method is limited to a small weight percent of zirconium. When higher amounts of zirconium are mixed with cement grout, ignition of the material may still occur. Utilization of approved mixing ratios of zirconium-to-cement grout results in significant volume and weight increases of the waste form.
Utilization of cement grout has been considered acceptable by the EPA. Although cement is still commonly used for solidifying radioactive waste there is some movement by the Nuclear Regulatory Commission to restrict its use due to strength and radiolysis (particularly hydrogen generation) concerns under high radiation doses.
Oxidation of non-radioactive zirconium swarf is commonly used by zirconium producers to treat the swarf prior to land disposal. Although this process is effective; however, it is costly to comply with safety requirements and meet EPA regulations to control off-gas emissions. Oxidation of radioactive zirconium would require even more costly controls to ensure safety and preclude the emission of radioactive off-gas particulate.
Limited information is found in the literature related to stabilizing zirconium cutting swarf. Most of the information available is related to solidification with hydraulic cements (such as portland cement, gypsum, and concrete) that harden by chemical reaction with water.
A solid matrix has been used to dispose of irradiated zirconium swarf generated in the process of decladding spent fuel from the Enrico Fermi reactor. Swarf accumulated from grinding-off the zirconium cladding (with some diffused uranium) of approximately 420 fuel pins was solidified in several gypsum.castings and disposed of at the Nevada Beatty Site. Testing of castings with non-radioactive zirconium swarf showed that the ignitable propensity of the zirconium was stabilized when the castings contained less than 5 weight percent zirconium.
A cement grout solidification process has also been used to dispose of swarf composed of zirconium cladding and uranium oxide. The swarf was generated when sectioning a test reactor core with an abrasive cut-off saw. Testing of castings with non-radioactive zirconium swarf showed that the ignitable propensity of the finely divided zirconium was stabilized in a casting containing 1 weight percent zirconium. The solidified waste form was disposed of at the Idaho National Engineering Laboratory (INEL) Radioactive Waste Management Complex.
Encapsulation of magnesium swarf generated when mechanically stripping-off Magnox (magnesium alloy) fuel cladding from the British gas cooled reactor fuel has also been used. Magnesium has an ignitable propensity comparable to zirconium and other reactive metals such as aluminum, titanium, uranium, etc. when in a finely divided form. A plant was built to process the Magnox swarf with cement grout and blast furnace slag.
It has been reported that solidified concrete castings of a mixture of zirconium and uranium swarf ignited as the castings solidified during the curing process. Ignition of the castings was attributed to curing the castings in a hot building and due to the heat of hydration generated as the casting cured. Subsequent testing has shown that the castings would ignite with mixtures containing approximately 20 weight percent of uranium metal.
To prevent ignition of zirconium swarf, it is often stored submerged in water. This has made the use of hydraulic cements attractive as a solidification binder since the water used to store the swarf in can be used to mix and harden the cement. However, if the swarf is stored with excessive amounts of water, the swarf must be removed from the water or at least partially drained. The primary disadvantage of using hydraulic cements is the increased volume of the waste form. The Environmental Protection Agency (EPA) initially considered solidification of radioactive zirconium swarf with cement as impermissible dilution since they had been processed with relatively low waste concentrations. After public comments and further review, cement solidification was accepted as a viable technology for stabilizing radioactive zirconium swarf
An alternate solidification binder was sought that would generate less waste volume than hydraulic cements and which could be injected directly into zirconium swarf waste which is submerged under several feet of water. By maintaining the water coverage over radioactive swarf, radiation exposure to personnel processing the waste is minimized and the swarf is protected from ignition sources. The potential for the casting igniting due to the heat generated by the exothermic reaction of the curing process is eliminated by using an underwater process. Underwater solidification minimizes handling of the waste and utilizes the water to provide an inherently safe work condition. Such an underwater process was unavailable; consequently, work was begun to determine if such a process could be developed.
It was discovered that solidification binders, such as hydraulic cements, would be unacceptable for underwater processing. It was desired to have a low viscosity liquid binder which would penetrate the waste and then solidify underwater. It was found that vinyl ester-styrene had properties attractive for this application but had never been used in an underwater process.
Dow Chemical began the development of vinyl ester-styrene (VES) as a solidification binder for radioactive waste in the 1970's. Dow performed extensive testing to get it accepted by U.S. regulatory agencies and radioactive waste burial sites. Dow eventually discontinued marketing efforts due to the relatively low cost of radioactive waste burial at the time and the introduction of High Integrity Containers which were more competitive for radioactive waste disposal. Later when radioactive waste burial costs escalated, Diversified Technologies began using VES for processing radioactive ion exchange resin beads and sludge wastes. Diversified Technologies processes waste for commercial nuclear power plants using VES and has promoted using the product for other waste applications. EG&G Idaho, Inc., performed waste form testing for the Department of Energy and the Nuclear Regulatory Commission to show that VES would meet the regulatory requirements to solidify ion exchange resins from the clean-up of the damaged Three Mile Island reactor.
U.S. Pat. No. 4,585,583, ROBERSON et al., relates to solidifying ion exchange resin beads using a solidification binder formulation comprising vinyl ester-styrene and a benzoyl peroxide catalyst, and an in-situ process that blends the solidification binder into the waste without mixing.
U.S. Pat. No, 4,297,827, ALLISON et al., relates to a solidification process to prepare radioactive waste for dumping in the ocean and is directed toward solidifying ion exchange resin beads.
U.S. Pat. No. 3,723,338, GODFREY et al., relates to a solidification process to immobilize contaminants in granular media such as soil.
U.S. Pat. No. 5,318,730, RIESER et al., relates to a spray process to coat the exterior of a waste, and more specifically is directed toward preparing large contaminated items that must be shipped or stored temporarily prior to disposal.
U.S. Pat. No. 4,851,155, KANAGAWA et al., relates to solidifying granular radioactive material using a vinyl type monomer.
U.S. Pat. No. 4,834,917, RAMM et al. relates to encapsulating a waste by compressing a copper or ceramic powder around the waste.
U.S. Pat. No. 4,929,394, KITAGAWA et al., relates to compacting radioactive metal wastes to prepare them for disposal.
U.S. Pat. No. 4,859,395, UNGER et al., relates to stabilizng hazardous wastes by encapsulating the waste with thin layer of thermosetting plastic using a three stage process.
Prior to the present invention, however, it is not believed that there was an acceptable process to dispose of zirconium swarf underwater or without greatly increasing the waste volume.